🚀 go-pugleaf

Eric Smith  wrote:
>
>My first question to Physhare!  I'm a young Physics teacher and this
question
>posed by a student boggled me.
>"We typically explain that objects cannot reach the speed of light
because
>mass increases toward infinity.  But if this is true, shouldn't the
force of
>gravity on that object increase to infinity, making c reachable?"
>What am I missing?
>
   In special relativity momentum must be defined as

        p = mv/sqr root(1 - v^2/c^2)

if momentum is to be conserved and if the relationship

        f = dp/dt

is to remain true.

    To make the expression for momentum look like the
classical definition physicists began defining a so-called
relativistic mass as

      relativistic mass = m/sqr root(1 - v^2/c^2)

Then momentum could be defined as

       ralativistic mass x velocity

making the definition of momentum look more like the classical
definition.

   But speaking of relativistic mass is just a way of writing
the momentum equation more conveniently - there is no real reason
why the rest mass should be grouped with the square root to define a
new mass. Rather than saying the mass changes in special relativity,
we can also just say the definition of momentum changes. In fact,
not equation from classical mechanics can be converted to its
relativistic equivalent simply by inserting the relativistic mass for
the rest mass.

  But if speaking in terms of relativistic mass is just a
question of convenience, there really is no change in mass
as the speed increases. The mass would still be the rest mass
and it would be the rest mass which still determines the force
on the object from any external gravitational field.

  Hope I made what I'm trying to say clear,

Ed Schweber
e-mail: EdSchweb@ix.netcom.com